System and method for receiving telemetry data from an implantable medical device

ABSTRACT

A system and method for receiving telemetry data from implantable medical devices such as cardiac pacemakers with improved noise immunity is disclosed. Ambient noise levels and signal strength are monitored and used to adaptively adjust the detection sensitivity of the receiver. Filtering of the received signal is performed to remove both broadband and narrowband noise. Removal of narrowband noise is accomplished with notch filters that are dynamically adjusted in accordance with a detected noise spectrum.

FIELD OF THE INVENTION

This invention pertains to implantable medical devices such as cardiacpacemakers and implantable cardioverter/defibrillators. In particular,the invention relates to a system and method for receiving telemetrydata from such devices.

BACKGROUND

Implantable medical devices, including cardiac rhythm management devicessuch as pacemakers and implantable cardioverter/defibrillators,typically have the capability to communicate data with a device calledan external programmer via a radio-frequency telemetry link. One use ofsuch an external programmer is to program the operating parameters of animplanted medical device. For example, the pacing mode and otheroperating characteristics of a pacemaker are typically modified afterimplantation in this manner. Modem implantable devices also include thecapability for bidirectional communication so that information can betransmitted to the programmer from the implanted device. Among the datawhich may typically be telemetered from an implantable device arevarious operating parameters and physiological data, the latter eithercollected in real-time or stored from previous monitoring operations.

Noise refers to any unwanted signal that interferes with thetransmission and processing of data signals in a communications system.Such noise may arise from sources either internal or external to thesystem. Because of limited energy storage capability, implantablemedical devices must necessarily transmit their data with a low signalenergy, making the transmissions very susceptible to interference fromnoise. This means that an external programmer can only be satisfactorilyused to receive data in relatively noise-free environments. Because ofthe widespread nature of electromagnetic noise sources, such aconstraint may not only be inconvenient to the patient and clinician,but could also be hazardous in an emergency situation. Both broadbandand narrowband noise sources contribute to the problem, with modern CRTmonitors being a particularly common source of narrowband noise.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a system andmethod for receiving telemetry data from an implantable medical devicewith an improved capability for operating in noisy environments. Inaccordance with the invention, a telemetry data receiving systemreceives a radio-frequency signal transmitted from an implantablemedical device with an antenna suitable for positioning in proximity tothe device. In one embodiment, the transmitted signal is a carrierwaveform modulated with digitally encoded data in the form of transmitpulses. The received signal is digitized into input signal samples thatare input to a matched filter having filter coefficients that correspondto a transmit pulse. A pulse detector compares output values of thematched filter with an adaptive pulse threshold value in order to detectthe presence of transmit pulses within the input signal samples.Adaptation is implemented by a threshold adjustment routine whichadjusts the pulse threshold value in accordance with measured peakvalues of both noise and the transmitted signal.

In a particular embodiment, narrowband noise is removed from the inputsignal samples with a series of notch filters having center notchfrequencies generated adaptively so that the notch frequencies match thefrequency peaks of a detected noise spectrum. The noise spectrum isdetected by first computing a power spectrum of the input signal andthen subtracting from it a template spectrum corresponding to anexpected input signal without noise. A template spectrum is computedfrom a representative input signal generated under noise-free conditionsso that when it is subtracted from the input signal spectrum, the resultapproximates the power spectrum of the narrowband noise alone. In orderto produce a detected noise spectrum that most closely approximates thetrue noise spectrum, the template spectrum is scaled by a factor thatreduces the total power in the detected noise spectrum to a minimalvalue. The frequency peaks in the detected noise spectrum are thenidentified and used to synthesize filters with corresponding notchfrequencies to remove the noise from the input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a telemetry system for an external programmer.

FIG. 2 is a diagram of the receiver portion of the telemetry system.

FIG. 3 is a diagram of the signal processing functions performed by thereceiver.

DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT

Telemetry systems for implantable medical devices utilizeradio-frequency energy to enable bidirectional communication between theimplantable device and an external programmer. An exemplary telemetrysystem for an external programmer and a cardiac pacemaker is describedin U.S. Pat. No. 4,562,841, issued to Brockway et al. and assigned toCardiac Pacemakers, Inc., the disclosure of which is hereby incorporatedby reference. A radio-frequency carrier is modulated with digitalinformation, typically by amplitude shift keying where the presence orabsence of pulses in the signal constitute binary symbols or bits. Theexternal programmer transmits and receives the radio signal with anantenna incorporated into a wand which can be positioned in proximity tothe implanted device. The implantable device generates and receives theradio signal by means of an antenna, such as may be formed by a wirecoil wrapped around the periphery of the inside of the device casing. Asaforesaid, the limited energy storage capability of typical cardiacrhythm management devices necessitates that the signals transmitted fromthe implantable device be of low energy, thus decreasing thesignal-to-noise ratio of the signal received by the external programmer.

The present invention is a telemetry data receiving system for anexternal programmer which offers improved performance in the presence ofnoise, a particular implementation of which is described below. Datagenerated by the implantable device is transmitted in the form of acarrier signal modulated with transmit pulses representing the encodeddata. In this embodiment, the received signal is digitized into inputsignal samples, and noise is removed from the samples by two filteringoperations implemented in the digital signal processor, one fornarrowband noise and the other for broadband noise. A series of infiniteimpulse response (IIR) notch filters is used to remove narrowband noisefrom the transmitted signal with the filter coefficients dynamicallygenerated in accordance with a detected narrowband noise spectrum.(Other embodiments may utilize FIR or analog filters to remove thenarrowband noise.) Narrowband noise, such as that generated by certainelectronic devices, is bandwidth-limited noise having a power spectrumwith characteristic frequency peaks. Thus, a series of notch or bandstopfilters with notch frequencies that correspond to those characteristicfrequency peaks will remove the narrowband noise from an input signal inreal time. Because the power spectrum of narrowband noise found in theenvironment is not constant, however, successful removal of such noiserequires that the notch frequencies adapt to a changing noise spectrum.In accordance with the invention, a power spectrum corresponding tonoise present within an input signal is detected by subtracting atemplate spectrum from the power spectrum of the input signal. Thedetected noise spectrum is then used to synthesize the notch filtersthat remove the noise from the input signal. By continuously orperiodically detecting a noise spectrum from the input signal, the notchfilters can be resynthesized with updated notch frequencies in nearreal-time to adaptively remove noise from the input signal in responseto a changing noise spectrum. A finite impulse response (FIR) matchedfilter then correlates the input signal with a signal corresponding to atransmit pulse in order to remove broadband noise. (In otherembodiments, matched filtering can be performed with an IIR or analogfilter.) Further noise immunity is provided by dynamically adjusting thethreshold at which pulses are detected from the output of the matchedfilter in accordance with measured noise and signal peaks.

FIG. 1 is a block diagram of the telemetry system of an externalprogrammer. The telemetry processor 50 supervises the operation of thetelemetry system, processes the data generated by it, and handlesprotocol functions such as timing, serial-parallel conversions, andcyclic redundancy code (CRC) checks. The telemetry processor 50communicates with the main host processor 30 of the programmer over ahost bus 40. The telemetry digital signal processor (DSP) 100 performsmost of the basic processing functions for the telemetry system. Itcontrols the transmitter, monitors the ambient noise level, and mayperform some protocol functions. As described below, the DSP 100 is alsoresponsible for matched filtering of the input samples, creating optimalnotch filters for removing narrowband noise in the local noiseenvironment, and dynamically adjusting the threshold signal level atwhich pulses are detected. A configuration and status channel 214between the DSP and telemetry processor allows the telemetry processorto configure the telemetry system for a particular implantable device,monitor the received signal strength, set automatic or fixed transmitterpolarities, read the wand status (i.e., presence and type), and updatethe DSP firmware.

The transmitter portion of the telemetry system is controlled by the DSPand includes a transmitter power supply 212, a power driver 210, and atransmit filter 208. The transmitter power supply provides voltages thatare compatible with the telemetry wand antenna and providesadjustability of the transmit power by the DSP. The power driver iscontrolled by the DSP and generates square waves that minimizeinterference with surface ECG and pace detection. The transmitter filterremoves high-frequency components of the power driver's waveform thatmay cause radiative interference with other devices. A wand antenna 205is used for both transmitting and receiving signals. The wand styledetector 206 senses both the presence of a wand and the wand type bymeasuring the resistance of a wand identification resistor. This allowsthe telemetry system to adjust the transmitter and receiver as necessaryfor particular types of wand antennas. The detector also causes thesystem to disable the transmitter if the wand is disconnected. Theanalog portion of the receiving circuitry includes a filter/amplifier204 that amplifies signals received by the wand as necessary and appliesthe low-pass anti-aliasing filtering to the signal prior toanalog-to-digital conversion by A/D converter 202. The DSP controls thefilter/amplifier's overall gain to adjust for the responses of differenttypes of wands.

FIG. 2 is a block diagram of the components making up the receiverportion of the telemetry system. The wand antenna 205 transduces achanging magnetic field intensity to a voltage which is the input signalto the analog receiver circuitry. The filter/amplifier 204 includes gaincircuitry 204 a that is distributed throughout the receiver and iscontrollable by the DSP, and a filter 204 b that provides ananti-aliasing function with its poles distributed throughout the analogreceiver circuitry. In an exemplary embodiment, a 100 KHz carrier signalis ASK modulated with a pulse train sub-carrier encoded with digitaldata, and the transmit pulses occur at a typical rate of 4 KHz with apulse width between 20 and 100 microseconds, resulting in a bandwidth ofthe modulated carrier of approximately 10 to 150 KHz. In order todigitally demodulate the carrier waveform, the analog-to-digitalconverter must then sample the received signal at a rate at least equalto the Nyquist frequency of 300 KHz. In order to provide goodcorrelation peaks in the matched filter used to detect transmit pulsesand to simplify the DSP code, the analog-to-digital converter shouldpreferably sample at a somewhat higher rate (e.g., approximately 350-400KHz). The resolution of the A/D converter should also be at least 10bits in order to provide dynamic range without an automatic gain controlcircuit. In an exemplary embodiment, a 150-kHz, seventh-orderButterworth filter provides the anti-aliasing function prior tosampling, and a 10-bit analog-to-digital converter (ADC) 202 withintegrated sample and hold generates the input signal samples. Afeedback mechanism within the analog receiver regulates a voltage biasto the receiver input which tends to remove any low frequency componentsfrom the input signal. The output of the ADC is a synchronous serialdata stream which is sent to the DSP, and the DSP controls the samplerate of the ADC.

FIG. 3 is a block diagram of the functions performed by the digitalsignal processor 100. When the DSP receives a sample from the ADC, aninterrupt is generated. The receiver interrupt handler 110 executed bythe DSP processes the samples in the time domain with notch filters 112and a matched filter 113, digitizes the presence or absence of transmitpulses via pulse detector 114, and then sends this digital data signalTEL_RX to the telemetry microprocessor 50. The receiver interrupthandler also fills a 512 element noise buffer 115 with consecutive rawinput samples. When this buffer is filled, the filter generator task 120processes the buffered data to generate new notch filter coefficients.The receiver interrupt handler then uses these coefficients toadaptively filter out narrowband noise. A noise peak detector 116 and asignal peak detector 117 detect and save peak signal values and peaknoise values, respectively. These peak values are periodically processedby the threshold adjustment task 130 in order to adaptively set thethreshold that the pulse detector 114 uses to digitize the serialstream.

An integer conversion routine 111 initially subtracts an offset from theinput sample to convert the sample from an unsigned integer to a signedinteger and remove any bias added by the analog receiver. The sample isthen processed through a six biquad IIR filter 112. Each biquad iseither a notch filter or a simple pass-through function so that zero tosix notch filters may be active at any time. The purpose of the notchfilters is to remove narrowband noise from the input signal samples.Since the presence and frequency of this noise depends on the ambientenvironment, the notch filter coefficients are adaptively generated inresponse to detected narrow band noise. The filter generator task 120does this by processing the raw input data in the buffer 115 andperiodically updating the IIR filter coefficients.

In order to obtain an optimum frequency response characteristic, thenotch filters in this implementation are recursive filters (i.e.,infinite impulse response) with adaptively generated filter coefficientsso that the notch frequencies match the frequency peaks of a detectednoise spectrum. The noise spectrum is detected by first computing apower spectrum of the input signal. The receiver interrupt handler 110fills a 512 element buffer 115 with consecutive raw input samples. Whenthis buffer is full, this task then scales the buffer values up to limitround-off noise in later calculations at block 121, applies a windowingfunction such as a Hamming window to the data to limit spectralspreading at block 122, and then discrete Fourier transforms the timedomain data into frequency domain data via a Fast Fourier Transform(FFT) algorithm at block 123. The FFT output is then transformed into apower spectrum by taking the norm of the FFT output at block 124. Thereceiver interrupt handler then fills the buffer again, and the mean ofeight consecutive power spectra is taken by block 125. This averagepower spectrum is then processed by noise spectrum detector 126 in orderto detect narrow band noise peaks by subtracting from it a templatespectrum corresponding to an expected input signal without noise. Atemplate spectrum is pre-computed from a representative input signalgenerated under noise-free conditions so that when it is subtracted fromthe input signal spectrum, the result approximates the power spectrum ofthe narrowband noise alone. In order to produce a detected noisespectrum that most closely approximates the true noise spectrum, thetemplate spectrum is scaled by a factor that reduces the total power inthe detected noise spectrum to a minimal value. The frequency peaks inthe detected noise spectrum are then identified and used to synthesizefilters with corresponding notch frequencies to remove the noise fromthe input signal. The notch filters are synthesized with well-knownfilter synthesis algorithms by filter synthesizer 127.

An exemplary implementation of the filter coefficient updating methodjust described is as follows. Let P_(i) be the power spectrum of theinput signal and P_(e) be the template spectrum corresponding to thenoise-free signal. The detected noise spectrum P_(n) is then computedas:

P _(n) =P _(i) −R*P _(e)

where R is a scaling factor chosen to minimize P_(n). As a firstapproximation, R is set to a ratio of P_(i) to P_(e) computed bydividing P_(i) by P_(e) for each frequency bin, totaling up theseratios, and taking the average ratio. A successive approximationapproach is then used to find the optimal scaling factor. First, valuesfor R are found that produce a positive P_(n) and a negative P_(n),referred to as R+ and R−, respectively. Since between these two valuesis the value of R that minimizes P_(n), new values for R are computed asthe average of R+ and R−. As each new R value is tried in the aboveequation, it replaces the previous value of R+ or R− according towhether P_(n) is made negative or positive, respectively. The procedureis iterated until the optimal value of R is found to result in the noisespectrum P_(n). Spectral threshold values for setting the notch filtercoefficients are determined by computing the mean and standard deviationof the spectrum P_(n). In a preferred embodiment, the spectralthresholds are then set at three standard deviations above the mean.These spectral thresholds then constitute the frequency peaks used toset the notch frequencies of the notch filters 112.

Referring to block 110 of FIG. 3, the output of the notch filter stageis input to the FIR matched filter 113. The coefficients of the FIRfilter 113 are designed to correlate the filtered input signal sampleswith samples that would be expected from a transmit pulse generated bythe implantable device. This type of filter is very effective indiscriminating transmit pulses from background noise and increases therange of the telemetry system. The FIR coefficients are derived bycapturing a strong, noise-free transmission signal from the implantabledevice immediately after the samples are converted to signed integers inthe receiver interrupt handler. The captured data is then manipulated sothat the signal samples are reversed in their order, thus flipping themin time, and each sample is amplitude offset so the average of thesamples is near zero in order to eliminate any DC component from thecoefficients. The samples are then normalized so that they arefractions, with the maximum sample amplitude equal to 1.0. Thesefractions are then scaled so the results are in the range of −32768 to32767 and then copied into the appropriate FIR coefficient table. Withthese FIR filter coefficients, the matched filter 113 performs aconvolution between the input signal samples and samples correspondingto a time-reversed version of the transmit pulse expected to begenerated by the implantable device, which is equivalent to performing across-correlation between the input signal and a transmit pulse. Theoutput of the matched filter 113 is then compared to a threshold value(td_threshold) by the pulse detector 114. The TEL_RX signal is set highif the filtered value is above td_threshold, otherwise TEL_RX is setlow.

The FIR filter output noise and signal peak values are calculated bypeak detectors 116 and 117 which are then saved for processing by thethreshold adjustment task 130. Signal values are discriminated fromnoise values based on the timing of the sampled data relative to thelast transmit pulse. If a sample occurs at a time when the telemetryprotocol does not allow a transmit pulse from the implantable device,then the sample is assumed to be noise, otherwise it is assumed to be asignal. These peak values are periodically processed by the thresholdadjustment task 130 in order to adaptively set the value of td_thresholdthat the pulse detector 114 uses to digitize the serial stream.

The threshold adjustment task 130 uses the peak noise and signal valuescalculated by the receiver interrupt handler to update the value oftd_threshold. The threshold is dynamic so that best spurious noiserejection is accomplished in noisy environments and maximum sensitivityis accomplished in noise-free environments. A local variable,min_threshold, is maintained. This variable is used to set a lower limitto the value of td_threshold. It can rapidly increase in value, but canonly slowly decrease in value. If the noise peak is greater thanmin_threshold as determined at step 131, then min_threshold is assignedthe noise peak value at step 132. This is done so that spurious noisewhich makes its way through the digital filters can be rapidly respondedto. Note that Gaussian noise will statistically attain very large valueson rare occasions, so min_threshold will track to the Gaussian noisepeaks, not the average level. If the noise peak is not greater thanmin_threshold, then the peak value is averaged into min_threshold usinga weighted moving average at step 133. The peak value is lightlyweighted, so that the decay rate of min_threshold is relatively slow.This slow averaging is done because min_threshold is designed to guardagainst spurious noise conditions (i.e., noise absent over a shortinterval does not necessarily mean the noise has gone away).

The signal peak is then compared to min_threshold at step 134. If it isbelow min_threshold, then a transmitted signal is assumed to be absent,and td_threshold simply remains at its current value. If the signal peakis above min_threshold, then the signal peak value is averaged into alocal store using a weighted moving average at step 135. The value oftd_threshold is then set to half the value of this local store. Thus,the value of td_threshold tends to be one half the value of the transmitpulse peak value. The signal peak is weighted relatively heavily, sothat td_threshold can react to variations in telemetry range thatnormally occur as the operator manipulates the wand. Note that if thecalculated value of td_threshold is below min_threshold, thentd_threshold is clamped to the value of min_threshold. Also, although itis desirable to rapidly change td_threshold in order to react to rangevariations, it is not desirable for it to change too quickly. Since thematched filtered transmit pulses have a finite slope, the threshold atwhich a signal is detected will affect the time at which the digitizedoutput changes state. Since the time domain FIR filter output tends tohave a shape similar to a Sinc function, rapid variations intd_threshold could detect only the main lobe for some data bits, andleading side lobes for other data bits. Certain synchronizationprotocols are particularly sensitive to this problem, since they use analignment bit to establish the data window timing for the subsequentdata bits. The threshold should therefore preferably be stable from whenthe alignment bit is detected to when the last data bit is detected.Thus, the averaging weight of the peak signal is preferably chosen toachieve the best compromise between responsiveness to range variationand threshold stability during receipt of transmitted word.

In the embodiments of the invention described above, the received signalwas digitized and processed in the digital domain to derive the transmitpulses. In other embodiments, the received signal could be processed inthe analog domain to remove narrowband and narrowband noise, correlatethe signal with a transmit pulse by matched filtering, and detecttransmit pulses with an adaptive threshold.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A system for receiving data from an implantablemedical device, comprising: an antenna for receiving an input signalwhich includes a transmitted signal containing transmit pulses from theimplantable device; a matched filter for filtering the input signal andproducing an output signal which corresponds to a cross-correlationbetween the input signal and a transmit pulse expected to be generatedby the implantable device; a noise peak detector for measuring a peaknoise value in the input signal; a transmitted signal peak detector formeasuring a peak transmitted signal value in the input signal; circuitryfor adjusting a pulse threshold value which computes the pulse thresholdvalue as a value between the measured peak noise value and the measuredtransmitted signal peak value; and, a pulse detector for detectingtransmit pulses within the input signal by comparing the output signalof the matched filter with the pulse threshold value.
 2. The system ofclaim 1 further comprising an analog-to-digital converter for digitizingthe received input signal into input signal samples which are input tothe matched filter and wherein the matched filter, noise peak detector,transmitted signal peak detector, pulse threshold adjustment circuitry,and pulse detector are implemented in the digital domain.
 3. The systemof claim 2 wherein the implantable device utilizes a telemetry protocolwhich specifies when transmit pulses are allowed to be transmitted andfurther wherein the transmitted signal peak values are measured when theimplantable device is known to be transmitting transmit pulses inaccordance with the telemetry protocol and the noise peak values aremeasured at times when the implantable device is known not to betransmitting transmit pulses in accordance with the telemetry protocol.4. The system of claim 3 wherein the noise peak detector and thetransmitted signal peak detector detect peak values of the matchedfilter output signal in order to measure the noise peak value and thetransmitted signal peak value, respectively.
 5. The system of claim 4wherein the pulse threshold adjustment circuitry computes the pulsethreshold value by performing the following steps: adjusting a storedminimum pulse threshold value by comparing a current stored minimumpulse threshold value with a current measured noise peak value, settingthe minimum pulse threshold value equal to the current measured noisepeak value if the current measured noise peak value is greater than thecurrent minimum pulse threshold value, and setting the minimum pulsethreshold value equal to a weighted moving average of the current andprevious measured noise peak values if the current measured noise peakvalue is less than the current minimum pulse threshold value; and,leaving the pulse threshold value unchanged if the current measuredtransmitted signal peak value is below the current minimum pulsethreshold value and setting the pulse threshold value equal to aspecified fraction of a weighted moving average of the current andprevious measured transmitted signal peak values otherwise.
 6. Thesystem of claim 5 wherein the moving average of the current and previoustransmitted signal peak values is computed by the pulse thresholdadjustment circuitry with the current transmitted signal peak valueweighted more heavily than the previous transmitted signal peak values.7. The system of claim 6 wherein the filter generator detects a noisespectrum by applying a Hanning window to the stored input signal samplesto limit spectral spreading prior to performing a discrete Fouriertransform.
 8. The system of claim 7 wherein the filter generator furtherdetects a noise spectrum by taking the norm of the discrete Fouriertransformed input signal samples.
 9. The system of claim 8 wherein thefilter generator further detects a noise spectrum by averaging aplurality of discrete Fourier transformed sets of input signal samplesstored in the noise buffer.
 10. The system of claim 9 wherein the filtergenerator further detects a noise spectrum by scaling the pre-computedtemplate spectrum by a factor that reduces the total power in thedetected noise spectrum to a minimal value.
 11. The system of claim 4wherein the noise peak detector and the transmitted signal peakdetectors discriminate transmitted signal values from noise values basedon the timing of the matched filter output signal relative. to the lastdetected transmit pulse.
 12. The system of claim 5 wherein the specifiedfraction of the weighted moving average of measured transmitted signalpeak values is one-half.
 13. The system of claim 5 wherein the movingaverage of the current and previous noise peak values is computed by thepulse threshold adjustment circuitry with the current noise peak valueweighted less heavily than the previous noise peak values.
 14. Thesystem of claim 2 wherein the pulse threshold adjustment circuitrycomputes the pulse threshold value as a specified fraction of a weightedmoving average of measured transmitted signal peak values that exceed aspecified minimum pulse threshold value.
 15. The system of claim 14wherein the pulse threshold adjustment circuitry adjusts the minimumpulse threshold value in accordance with measured noise peak values. 16.The system of claim 15 wherein the pulse threshold adjustment circuitryadjusts the minimum pulse threshold value by comparing a current minimumpulse threshold value with a measured noise peak value, setting theminimum pulse threshold value equal to the measured noise peak value ifthe measured noise peak value is greater than the current minimum pulsethreshold value, and setting the minimum pulse threshold value equal toa weighted moving average of measured noise peak values if the measurednoise peak value is less than the current minimum pulse threshold value.17. The system of claim 16 wherein the pulse threshold adjustmentcircuitry leaves the pulse threshold value unchanged if the currentmeasured transmitted signal peak value is below the current minimumpulse threshold value and sets the pulse threshold value equal to thespecified fraction of the weighted moving average of measuredtransmitted signal peak values otherwise.
 18. The system of claim 17further comprising one or more notch filters for removing narrowbandnoise from the input signal samples prior to being input to the matchedfilter.
 19. The system of claim 18 further comprising a filter generatorfor detecting a noise spectrum in the input signal samples and forupdating filter coefficients of the notch filters so that the notchfrequencies correspond to the detected noise spectrum.
 20. The system ofclaim 19 wherein the filter generator detects a noise spectrum bydiscrete Fourier transforming a set of the input signal samples toproduce a power spectrum of the input signal and subtracting apre-computed template spectrum therefrom, the template spectrumcorresponding to the power spectrum of a representative input signalwithout noise.
 21. The system of claim 20 wherein the filter generatorand notch filters are implemented in a digital signal processor andwherein the filter coefficients of the notch filters are periodicallyupdated by the filter generator.
 22. The system of claim 21 furthercomprising a buffer for storing a set of consecutive input signalsamples and wherein the filter generator detects a noise spectrum in theinput signal samples stored in the noise buffer.
 23. The system of claim20 wherein the filter generator sets the notch frequencies of the notchfilters to match those spectral components of the noise spectrum thatexceed a specified spectral threshold value.
 24. The system of claim 19wherein the filter generator synthesizes the notch filters as digitalinfinite impulse response filters with notch frequencies correspondingto the detected noise spectrum.
 25. The system of claim 2 wherein thesignal transmitted by the implantable device is a carrier signaldigitally modulated by amplitude shift keying with a pulse trainsub-carrier encoded with digital data in the form of transmit pulseswhere the presence or absence of a transmit pulse in the transmittedsignal constitutes a binary symbol.